1. Field of the Invention
The present invention relates to a natural rubber master batch, a production method thereof, and a natural rubber composition. More specifically, the present invention relates to a natural rubber composition having improved in processability, reinforcing property and abrasion resistance, a natural rubber master batch suitable for producing the natural rubber composition, and a production method of the natural rubber maser batch.
2. Description of the Prior Art
Generally, natural rubber is known to be excellent in mechanical properties, low heat build-up, and abrasion resistance. In addition, natural rubbers receive attention as environmentally friend materials. Natural rubbers contain non-rubber components having amide linkages (protein). It has been known that the non-rubber component in natural rubber has an aging resistant effect and a vulcanization accelerating effect. In contrast, the non-rubber component has a drawback of reducing the processability of natural rubber as compared with synthetic rubbers because it increases the viscosity of rubbers by molecular chain entanglement caused by hydrogen bonding between amide linkages.
In recent special applications such as a natural rubber product for medical use, is used natural rubber having its non-rubber component such as proteins removed to a considerable extent by centrifugal separation of latex (Japanese Patent Application Laid-Open Nos. 6-56902, 8-143606, 11-71408, 2000-19801, etc.). Such natural rubber, however, involves a problem of a lowered elastic modulus and a poor aging property because the non-rubber component having the aging resistant effect and the vulcanization accelerating effect is removed nearly completely.
As a production method of rubbers of excellent processability, generally known is a method using a wet master batch. In this method, a slurry is prepared in advance by mixing a filler such as carbon black and silica with water in a certain proportion and then finely dispersing the filler throughout water by mechanical force. The slurry is then mixed with a rubber latex, followed by addition of a coagulant such as acids, inorganic salts and amines to coagulate the mixture, and recovery and drying of the coagulated product (Japanese Patent Publication Nos. 36-22729 and 51-43851).
As compared with a wet master batch for synthetic rubbers, a wet master batch comprising natural rubber and a filler such as carbon black, silica, other inorganic filler, etc. shows a smaller improving effect on the processability and has a drawback of hardly attaining a good dispersion of the filler. When excessively applying a mechanical shearing force to obtain a slurry with a good dispersion, aggregates (structures) of filler are broken to decrease the reinforcing property and deteriorate the abrasion resistance.